Thirty to forty percent of the human population under age 40 develop an ocular refractive error requiring correction by glasses, contact lenses, or surgical means. Refractive errors result when the primary optical elements of the eye, the cornea and the lens, fail to image incoming light directly on the retina. If the image is focused in front of the retina, myopia (nearsightedness) exists. If the eye image is focused behind the retina, hyperopia (farsightedness) exists. The focusing power of the eye or any of the eye's individual components is measured in units called diopters.
Approximately 20% of the patients under 40 having vision defects cannot wear contact lenses because the contact lenses do not fit (become dislodged and/or are very uncomfortable), or they fail to provide the requisite optical correction, or both. In addition, many patients who currently wear contact lenses are not satisfied with the length of time they can wear their lenses and/or with the visual acuity their contact lenses provide.
Over age 40, the percentage of the population requiring vision correction dramatically increases because the crystalline lens of the eye becomes relatively inelastic. The quality of the tear film decreases and the problems encountered with existing contact lenses become much more common and acute.
Standard contact lenses are rotationally symmetrical and spherical and vault from the sclera and rest on the cornea. The human cornea, however, is an "asymmetrically aspheric" surface.
"Aspheric" means that the radius of curvature along a corneal "meridian" (which is an imaginary line on the corneal surface passing through the geometric center of the cornea, analogous to a geographic meridian) is not a constant. Indeed, the corneal curvature tends to flatten progressively from the geometric center to the periphery. "Asymmetric" means that the profile of the corneal curvature along a half-meridian is not the same as (i.e., it is not a mirror image of) the other half of the same meridian. Additionally, "asymmetric" means that the profile of the corneal curvature about a central point (i.e., an origin) is not the same as the corneal profile on the opposite side of the central point. The degree to which the cornea is aspheric and/or asymmetrical varies from patient to patient and within the same person.
Spherical lenses do not match the corneal curvature and geometry, and therefore do not fit properly. The more irregular the patient's cornea the worse the fit, such that about 20% of the patients under age 40 are unable to wear standard contact lenses.
Standard contact lenses are rotationally symmetrical. Sometimes the fitter will generate toric, bitoric and like surfaces in his effort to fit lenses on the cornea. These more complicated lens designs remain inherently rotationally symmetric, i.e., the surfaces are generated about a central point of revolution. Toric lenses are currently made in one of two ways. The first and most common method is to crimp and thus distort the lens blank before placing it in the lathe. After the crimped lens is cut, it is allowed to spring open. The second method is to make the toric lens directly on a lathe.
Because the human cornea has an asymmetrically aspheric surface, purely spherical lenses poorly match the corneal curvature and geometry. When the lens is designed as a toric lens, the resultant lens surfaces are still rotationally symmetrical (i.e., these lenses are not asymmetrical and aspheric). In some eyes the discrepancy between the lens and underlying cornea's asymmetry is so great that toric lenses fail to ride on the cornea and/or give satisfactory vision.
In an effort to alleviate these problems, manufacturers developed lenses with varying curvatures on their posterior surface. For example, U.S. Pat. No. 5,114,628 discloses aspherical contact lenses made using corneal topographic data to control a lathe. (The data, as taught in the '628 patent, provide information on the slope of the corneal surface at different points on the cornea and are based on measurements in two dimensions, interpreted three-dimensionally.) The resultant lens is aspherical (in both the anterior and posterior surface) but inherently symmetrical. Although, such a lens may fit some patients better than the standard spherical lenses, other problems such as increased weight and poor tear exchange under the lens can aggravate the patient's vision or comfort. But other patients may experience more discomfort than with the spherical lenses. Thus, this type of aspherical symmetric lens does not provide a substantial improvement in the number of patients that can comfortably wear contact lenses and/or wear contact lenses that provide them with the requisite visual acuity.
U.S. Pat. No. 2,264,080 to Hunter discloses a system for manufacturing a "contoured" scleral contact lens, i.e., a lens resting outside and intentionally vaulting the cornea. Hunter teaches the creation of a mold of the surface of the eye which is then used as a "template" to mechanically radially guide a grinder over the surface of a lens blank. The grinder receives information about the meridional topography of the mold and travels over the surface of the lens blank in a back-and-forth fashion along meridians of the lens. Hunter's scleral lens intentionally has sufficient clearance from the cornea to avoid any contact with the surface of the cornea. Moreover, his method of manufacture causes "ridges" or "cusps" to be formed on the posterior surface of the lens, which if present on a contact lens closely fitted to the cornea would possibly abrade and cause discomfort to the wearer. Additionally, these ridges would extend into the optical field portion of a contact lens, obstructing the patient's field of vision and thereby rendering the contact lens useless. Hunter intentionally avoids forming the anterior surface of the lens in any conformity to the central optical zone of the corneal surface.
Accordingly, there is a need in the art for a better fitting contact lens that will decrease or eliminate the number of patients of all ages who currently cannot wear contact lenses, and provide better comfort and/or visual acuity (including better correction of astigmatism) for patients who now wear contact lenses. U.S. Pat. Nos. 5,502,518 and 5,570,142 both to Lieberman (the present inventor), which are assigned to the same Assignee as the present invention, are both directed to contact lenses that have posterior surfaces that accurately match at least a portion of the surface of the cornea. The '518 and '142 patents satisfied the need for better fitting contact lens. The present invention is a further refinement of the '518 and '142 patents and provides increased acuity by dividing the surface of the lens into a plurality of segments, each of which has a relatively small surface area so that, particularly in the lens' optical region, the posterior surface of the lens will more closely conform to or match the surface of the underlying cornea resulting in negation of the lens effect of the tear film and, hence, improved acuity. The disclosures of U.S. Pat. Nos. 5,502,518 and 5,570,142 are hereby incorporated by reference in their entirety. In the case of inconsistencies, the present description, including definitions, will control.
The present inventor surprisingly discovered that the cornea, in most patients, is actually naturally tilted to a varying degree with respect to the pupillary axis of the eye. Additionally, the degree of corneal tilt varies within the individual cornea depending on the diameter over which the tilt is measured. More specifically, the intersection between the cornea and the sclera (i.e., the base of the cornea) is tilted with respect to a reference plane that is parallel to a tangent at the "high point" of the cornea. Thus, there is a need in the art to design a contact lens that accounts for this natural tilt of the cornea, and as a result fits better and provides better optic correction.
It is an object of the invention to provide a contact lens that accounts for the natural tilt of the cornea.
The present inventor also surprisingly discovered that the cornea is least asymmetrical about a particular point on the cornea which is furthest away on the Z axis with respect to a reference coordinate system, i.e., the "High Point". This point is not necessarily located in the either the cornea's or the contact lens' geometric center. It is an object of the invention to provide a contact lens that accounts for the "High Point", in order to provide better optic correction.
It is another object of the invention to rapidly and economically manufacture custom-fit contact lenses that provide increased visual acuity by aspherically and asymmetrically matching and/or conforming to a portion of the wearer's cornea.
It is another object of the invention to utilize the asymmetrical aspherical contact lens as an orthokeratology lens. More specifically, a posterior surface of a central optical portion of the lens is provided with a flatter shape to displace the cornea while an inner peripheral optical portion of the lens is recessed (i.e., steeper shape) to allow the cornea to bulge radially out in this annular region.
These goals are achieved using information obtained by surface modeling the cornea, and by manipulating this information to design a lens that not only conforms to the overall aspheric and asymmetric shape of the cornea but also takes the local geometry of the cornea into account, including the High Point and Tilt Axis. The process begins by initially scanning the cornea to generate a point cloud, as derived from the surface of the cornea. This point cloud together with the elevation of each point with respect to an arbitrary reference plane is then used to generate a corneal matching surface, preferably using computer 3-D modelling graphics. The high point of the generated corneal matching surface is then determined which corresponds to a point about which the cornea is least asymmetrical. Because refraction is customarily measured in 5.degree. increments, seventy-two (72) polar splines (5.times.72=360) are generated from the corneal matching surface, one at every 5.degree. interval. Each spline matches the shape of the underlying cornea at each 5.degree. interval. In the preferred embodiment, each of the splines originates from the high point and extends radially outwardly to a predetermined edge boundary of the contact lens that is being designed. Each spline is then divided into portions from which arcs can be created to model individual central and peripheral optic lens surfaces. The boundaries of individual surface segments are defined radially by splines and circumferentially by first or a first and second "drive rails", each of which is formed by the intersection of a cylinder and the corneal matching surface and the smaller one of which is enclosed within the larger one. Relatively small surface segments of the lens are thus created, by well known surface generating formulas, based on the known boundaries formed by the intersection of the arcs, splines and the first drive rail, the second drive rail and the bounded base of the lens. Orthokeratology lenses can also be made based on the same deformation.